Recently, considering environmental issues, attention has been paid to a vehicle employing a motor as a drive force source, such as an electric vehicle, a hybrid vehicle and a fuel cell vehicle. Such a vehicle includes a power storage unit implemented, for example, by a secondary battery or a capacitor for supplying electric power to the motor, and converting kinetic energy to electric energy during regenerative braking and storing such electric power.
In such a vehicle employing the motor as the drive force source, in order to enhance acceleration performance and running performance such as travel distance, greater charge/discharge capacity of the power storage unit is desired. Here, a configuration where a plurality of power storage units are mounted has been proposed as a method of increasing the charge/discharge capacity of the power storage unit.
For example, U.S. Pat. No. 6,608,396 discloses a power control system providing desired high DC voltage levels required by a high voltage vehicle traction system. The power control system includes a plurality of power stages for providing DC power to at least one inverter, each stage including a battery and boost/buck DC-DC converter, the power stages wired in parallel, and a controller controlling the plurality of power stages so as to maintain a voltage output to at least one inverter by causing uniform charge/discharge of the batteries of the plurality of power stages.
On the other hand, the drive force required in the vehicle significantly varies depending on a state of running. For example, during running at low speed or running down a slope, required electric power is small relative to the total value of allowable charge/discharge power in a plurality of power storage units. Then, in such a case, desirably, a voltage conversion operation of a voltage conversion unit (corresponding to the boost/buck DC-DC converter above) corresponding to a prescribed power storage unit is selectively stopped so that loss in electric power conversion in the voltage conversion unit is decreased.
In selectively stopping such a voltage conversion unit, a voltage conversion unit to be stopped is selected by reflecting a power storage state of the corresponding power storage unit. For example, a voltage conversion unit to be stopped is selected depending on magnitude of an output voltage of the power storage unit connected to each voltage conversion unit. Namely, a voltage conversion unit corresponding to a power storage unit smaller in an output voltage is preferentially stopped, so that generation of an unnecessary cyclic current between the power storage units is avoided.
In a power supply system having two power storage units relatively close to each other in capacity, an output voltage of each power storage unit can have a relatively close value. Accordingly, if a voltage conversion unit to be stopped is selected simply based on magnitude of the output voltage of the power storage unit, switching between the voltage conversion units to be stopped is made frequently and a supply voltage from the voltage conversion unit to a load device becomes unstable. In addition, each voltage conversion unit should repeat stop and execution of a voltage conversion operation so frequently that a control system involved with the voltage conversion operation becomes unstable.